Why is my nested while loop not working

Question

Working on a program in python to print out the first 1000 prime numbers (except 2). all i can get for the output is the number 3. don't understand where or when my loop is ending. very new at programming. can anybody help?

primeCounter = 1
candidate = 3

while primeCounter < 1000:
    isPrime = True
    counter = 2
    while counter < candidate:
        if candidate%counter == 0:
            isPrime = False
        else:
            counter = counter + 1

    if isPrime == True:
        print candidate
        primeCounter = primeCounter + 1

     candidate = candidate + 1

Have you tried debugging it at all? perhaps by having it output the values of the variables that are tested for conditionals and so forth? — Karl Knechtel
– Karl Knechtel, Commented Jan 3, 2012 at 3:57

qiao · Accepted Answer · 2012-01-03 03:48:01Z

3

primeCounter = 1
candidate = 3

while primeCounter < 1000:
    isPrime = True
    counter = 2
    while counter < candidate:
        if candidate%counter == 0:
            isPrime = False
            break # <<<<<<<<<<<<<<<<<   break here, or the loop will go infinite
        else:
            counter = counter + 1

    if isPrime == True:
        print candidate
        primeCounter = primeCounter + 1

    candidate = candidate + 1

answered Jan 3, 2012 at 3:48

qiao

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Comments

David Schwartz · Accepted Answer · 2012-01-03 03:48:17Z

2

Once you set isPrime to False, you never increment counter again, so you never get out of the inner while loop.

answered Jan 3, 2012 at 3:48

David Schwartz

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Comments

Valentin Briukhanov · Accepted Answer · 2012-01-03 05:27:28Z

0

Your problem in block

while counter < candidate:
    if candidate%counter == 0:
        isPrime = False

if not candidate % counter, you get infinity loop.

answered Jan 3, 2012 at 5:27

Valentin Briukhanov

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